Isolation circuits and filter circuits are often used in devices connected to transmission lines. The transmission lines carry a signal to the devices or away from the devices. In a network adapter card, for example, a combination isolation and filter circuit is used to transfer and filter a signal from the network cable to the adapter card, or from the adapter card to the network cable. The isolation part of the circuit isolates the rest of the adapter card circuitry from the network cable. The filter part removes high frequency components of the signal.
FIG. 1 illustrates two combination isolation and filter circuits 100 as used in a 10 MHz Ethernet adapter card. The transmit circuit 101 transmits signals from the adapter card to the network cable, while the receive circuit 102 receives signals from the network cable and transmits them to the adapter card. Part number FL1012 and FL1066, available from Valor, Inc., implements the isolation and filter circuits 100.
The transmit circuit 101 includes a filter 110, a transformer 120 and a common mode choke circuit 130. The filter 110 connects to the transmit side of the adapter card electronics and to the transformer 120. The transformer 120 connects to the common mode choke circuit 130. The common mode choke 130 connects to the network cable.
The filter 110 includes number of inductors and capacitor. The inductors and capacitors act as a multi-pole filter for signals being transmitted to the network. The frequency response of the transmit circuit 101 is shown in FIGS. 2A and 2B. The 100 MHz frequency response graph 210 shows that the transmit circuit 101 acts as a low pass filter, with a response that quickly rolls off near 16 MHz. The 1 GHz frequency response graph 200 shows that the transmit circuit 101 does not transmit more than -20 dB for any other frequency between 25 MHz and 1 GHz. For 10 Mbit Ethernet communications, such a frequency response is desirable because high frequency noise components of the transmitted signal are removed before the signal is transmitted to the network cable. The frequency response thus allows the adapter card to meet electromagnetic interference requirements.
Returning to FIG. 1, the transformer 120 electrically isolates the adapter card from the network cable. The transformer 120 has a relatively good frequency response. That is, both high frequencies (e.g., 800 MHz) and low frequencies (20 kHz) are passed through the transformer 120 with little attenuation.
The common mode choke 130 is part of the filter 110 and helps to remove high frequency components from the signal being sent to the network cable.
The receive circuit 102 includes circuits similar to those in the transmit circuit 101. However, the receive filter 112 has fewer inductors and capacitors than the filter 110, resulting in a low pass filter with fewer poles. Having fewer poles means that the receive filter 112 does not have as steep a roll off as the filter 110. Because the receive characteristics are not as stringent as the transmit characteristics, a filter having fewer poles is acceptable. Importantly, reducing the number of inductors and capacitors also reduces the cost of the filter 110.
In high volume manufacturing, saving only a few pennies per adapter card can save millions of dollars per year. The use of the isolation and filter circuit 100 adds a significant cost to the price of the adapter cards. Thus, it is desirable to have an isolation and filter circuit 100 with a lower cost but still maintain a similar frequency response.
Additionally, for different communications standards, a different part is needed to implement the different isolation and filter circuits 100. For example, one part will have the desired frequency response for 10 Mbit Ethernet, while another part will have the desired frequency response for 16 MHz token ring. Maintaining an inventory of all these different parts is expensive because, the individual cost of each of the parts is relatively expensive. Therefore, it is desirable to have a more versatile isolation and filter circuit 100.